Various techniques have been proposed to display the remaining amount of a compressed gas stored in a tank (see, for example, JP-A-2005-240854). The prior art technique disclosed in this cited reference JP-A-2005-240854 calculates the mass of a compressed gas fuel (compressed gas) stored in a tank from the pressure and the temperature of the compressed gas fuel and displays the remaining amount of the compressed gas fuel in the tank. Other known relevant techniques include those disclosed in JP-A-2004-63205, JP-A-59-31415, JP-A-2006-112492, and JP-A-2006-242839.
In the prior art technique disclosed in this cited reference JP-A-2005-240854, the outputs of, for example, a pressure sensor, a temperature sensor, and respective electronic components of a gas level display device include errors, which cause variations of the respective output values. Even when the actual mass of the compressed gas in the tank is unchanged but is constant, the calculated mass of the compressed gas in the tank has a variation (an increase or a decrease). This may undesirably cause the display of the remaining amount or the level of the compressed gas in the tank to include erroneous display, which indicates an increase of the level of the compressed gas in the tank, regardless of no additional supply of the compressed gas into the tank.